Describe the bug (Required)
When stopping a server, a configuration error is shown which may cause the server to hang forever without actually stopping

To Reproduce (Required unless not known)

Log output/error (if present)

[31Mar2021 15:50:11.464] [Server thread/ERROR] [net.minecraftforge.fml.config.ConfigFileTypeHandler/]: Failed to remove config ./world/serverconfig/whisperwoods-server.toml from tracker!
java.lang.NullPointerException: null
	at com.electronwill.nightconfig.core.file.FileWatcher.removeWatch(FileWatcher.java:134) ~[core-3.6.2.jar:?]
	at net.minecraftforge.fml.config.ConfigFileTypeHandler.unload(ConfigFileTypeHandler.java:76) ~[forge:?]
	at net.minecraftforge.fml.config.ConfigTracker.closeConfig(ConfigTracker.java:114) ~[forge:?]
	at net.minecraftforge.fml.config.ConfigTracker.lambda$unloadConfigs$2(ConfigTracker.java:88) ~[forge:?]
	at java.lang.Iterable.forEach(Iterable.java:75) ~[?:?]
	at java.util.Collections$SynchronizedCollection.forEach(Collections.java:2067) ~[?:?]
	at net.minecraftforge.fml.config.ConfigTracker.unloadConfigs(ConfigTracker.java:88) ~[forge:?]
	at net.minecraftforge.fml.server.ServerLifecycleHooks.handleServerStopped(ServerLifecycleHooks.java:135) ~[forge:?]
	at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:682) ~[?:?]
	at net.minecraft.server.MinecraftServer.lambda$startServer$0(MinecraftServer.java:232) ~[?:?]
	at java.lang.Thread.run(Thread.java:834) [?:?]

Expected behavior (Required)
For the server to stop cleanly and not hang with errors

Screenshots

Versions:

• Forge Version: 36.0.7
• MC Version: 1.16.5
• Mod Version: 2.0.0

Additional context
P.S. Your issue reporter tells users they're reporting for "Better Animals Plus", just a heads up!

This is usually caused by other mods. Let me know your pack or modlist. Do you have Genetic Animals installed?

Funny enough, I'm getting this same error from a lot of different mods. I don't have Genetic Animals installed, here's the full list:
mod_list.txt

Thanks for the heads up, the cause actually appears to have been the Performant mod, which in their latest update rewrote a lot of the config and doesn't appear to be backwards compatible with old ones, causing the server to hang on shutdown.
I'll file an issue report on their tracker so that people can track down the source of the error more easily.

Ah, performant screwing with my mods as usual! lol

TerraForged has also been associated with this issue, maybe try w/o that. What I do know is it's not whisperwoods most likely, as you should see if you remove WW it will error on another configuration file.